There are many circumstances in the oil industry where it is desirable to cut into or through downhole tubular goods within a well. For example, in the course of drilling a well, the drill pipe may become stuck at a downhole location. This may result from "keyseating" or as a result of cuttings which settle within the well around the lower portion of the drill string. In order to remove the drill string from the well, it may be necessary to sever the drill pipe at a location above the stuck point. Similarly, it is often necessary to carry out downhole cutting operatic, as during the completion, operation or abandonment of oil or gas wells. For example, it is sometimes desirable to sever casing or tubing at a downhole location in order to make repairs or withdraw the tubular goods from a well which is being abandoned. In other circumstances, it is desirable to perforate downhole tubular goods. Thus, it is a common expedient to perforate the casing and surrounding cement sheath of a well in order to provide fluid access to a hydrocarbon bearing formation. Similarly, it is sometimes desirable to perforate tubing in the completion or recompletion of a well.
As is well known in the art, chemical cutters can be used to significant advantage in the application of chemicals to cut, sever or perforate downhole tubular goods. For example, U.S. Pat. No. 2,918,125 to Sweetman discloses a downhole chemical cutter which employs cutting fluids that react violently with the object to be cut with the generation of extremely high temperatures sufficient to melt, cut or bum the object. In the Sweetman procedure, halogen fluorides are employed in jet streams impinging on the downhole pipe to sever or perforate the pipe. The attendant reaction is highly exothermic and the pipe is readily penetrated. Examples of chemical cutting agents disclosed in Sweetman are fluorine and the halogen fluorides including such compounds as chlorine trifluoride, chlorine monofluoride, bromine trifluoride, bromine pentafluoride, iodine pentafluoride and iodine heptafluoride. The cutting fluid is expelled from the tool through radial ports formed in the cylindrical wall of the tool in jet cutting streams. In Sweetman, the cutting ports extend radially from a central bore within the discharge head of the cutting tool which terminates in a reduced diameter bore which is open to the lower or front end of the cutting tool. The reduced diameter bore is internally threaded to receive a threaded plug which closes the lower end of the bore. A piston is slidably disposed in the central bore and is equipped with o-rings which bridge the cutting ports when the piston is in the uppermost position. The piston is driven downwardly during the cutting operation. Immediately above the cutting ports is an ignitor section which contains steel wool. The upper portion of the cutting tool is provided with anchoring assembly which functions to anchor the tool in response to an upward pull applied to the cable supporting the tool.
The normal practice in severing downhole tubular goods is to arrange the cutting ports in the cylindrical wall of the cutting head, as disclosed for example in U.S. Pat. No. 4,125,161 to Chammas. Here, the cutting head is a cylindrical member provided with a plurality of cutting ports arranged radially about the outer diameter of the cutting head. The cutting ports are bridged with a piston provided with o-rings to prevent the entry of fluids through the ports. A lower portion of the tool is provided with openings through which well fluid exerts hydrostatic pressure on the bottom of the piston, holding the piston in place before the tool is fired. The Chammas cutting tool incorporates an anchor sub having a plurality of wedges pivoted on an actuating piston near the upper end of the tool in which gas from a propellant charge displaces an actuating piston to cam the wedges outwardly against the tubing string or other object to be cut. The gas from the propellant charge is also employed to force the cutting chemical into contact with a pre-ignitor material and then outwardly through the cutting ports.
Where the downhole tubular goods are to be cut or formed of high strength corrosion resistant materials such as high chrome-nickel stainless steel, a chemical cutter may be employed in which the cutting parts are arranged in complimentary configurations to provide high-intensity streams of a cutting agent directed against the interior surface of the tubing or casing to be cut. For example, as disclosed in U.S. Pat. No. 5,320,174 to Terrell et at, a complimentary ring pattern formed of converging cutting ports can be employed to direct a high intensity cutting agent against the inner circumference of the casing or other tubular goods to be cut. Here the ignitor materials can take the form of a multi component accumulation such as steel wool having stainless steel chips intermingled within the steel wool.
Another downhole chemical cutting tool useful for cutting large diameter tubular goods is disclosed in U.S. Pat. No. 5,287,920 to Terrell. Here, the downhole chemical cutting tool is adapted to cut large diameter conduits downhole through the use of a cutting section having a plurality of externally upset cutting heads. These extend outwardly from the cylindrical cutting section to a point where they terminate in outer cutting surfaces having a desired effective diameter slightly smaller than the inner diameter of the tubular goods to be cut. Each of the cutting heads has a central chamber communicating with an interior chamber within the tool and a plurality of cutting ports which extend through the face of the cutting head from the interior chamber therein to the exterior of the cutting head. In a specific embodiment of this patent, the cutting heads are arranged in a spoke like configuration in which an outer disk portion is secured to the spoke by an enlarged threaded connection and the spoke is in turn threadedly secured to the tool body through a second, reduced threaded connection. Ignitor material may be positioned in the interior chamber within the tool located immediately below the section of the tool containing the chemical cutting agent or it may be located in the individual spokes or at both locations.
Yet another chemical cutting tool is disclosed in U.S. Pat. No. 4,494,601 to Pratt et al. Here, a lower part of the cutting head structure is open to well fluid and a piston plug is interposed immediately above the cutting ports. The cutting ports may be closed to the exterior of the well by means of an internal sleeve positioned in the bore of the cutting head immediately in front of the piston. When the tool is fired, the fluid pressure developed sets the anchoring means and forces the piston forward, exposing the port to the cutting fluid flowing into the bore from the chemical section. The tool further comprises means in the cutting section in front of the port to receive the piston upon the application of fluid pressure in the tool to lock the piston in place at a location in front of the cutting port. The locking means may take the form of a reduced section in the cutting tool bore which is adapted to receive a portion of the piston in a swedged relationship.